If you look at YPG, the Texans are 26th against the run and 12th against the pass. The passing stat is particularly misleading given that, for example, Big Ben didn't have the throw the ball for very many yards but did very effectively.

I'm not sure that the Texans should even think about being a top 10 defense until they even get mediocre in most meaningful defensive categories.

They are 18th for average yards per game given up.
They are 29th in sacks, with Mario getting 4 out of the 5.
They are 30th in points allowed.
Taking into account games played, they are 30th in total points
They are tied for 19th for forced fumbles
They are 23rd in interception

The good news is that if the offense shows more signs of life and the quarterbacks stop giving away the ball, the defense can improve to some degree. Sacks are easier to get if you get leads on the other team because it forces them to pass. (Personally, I don't like seeing teams forced to pass, because it so often leads to slant route, slant route touchdown).

The bad news is that the defense is performing worst than league average on a per play basis. See this entry here.

Ultimately rankings and stats are just something that are kind of interesting. The defense needs to make meaningful stops. They have problems on 3rd and 4th downs, and if you can't stop teams on that down, you are going to have a long day.